1. Field of Application
The present invention generally relates to the measurement of the optical characteristics of optical waveguide devices and, in particular, relates to devices for characterizing the optical effect of various devices upon an optical system.
2. Discussion of Prior Art
Waveguide devices such as Bragg gratings, interleavers, couplers, isolators, etc. require accurate characterization if they are to be used in deployable optical fiber networks. Generally this characterization has been accomplished using a few simple specifications such as insertion loss, bandwidth, polarization dependent loss, etc. The devices actually have responses that are significantly more complicated that these specifications can describe. Although the simplification of the characterization is necessary to allow for the use of interchangeable parts, and rational specifications for manufacturers to meet, obtaining a full and complete measurement of the device from which any desired specification can be calculated would have benefits to both manufacturers and their customers.
First of all, full device characterization would permit computer system models that are much more accurate, and would inherently contain aspects of the device that have not been explicitly specified. Second, in cases where manufacturers use different specifications, or consumers require a new set of specifications, devices could be accurately compared, or new specifications developed from the complete measurement function stored in a database. For most devices this measurement function would consist of four complex functions that would occupy less than 200 kB of storage. Finally, if the complete characterization of the device can be achieved with a single instrument, then final testing of devices would be greatly simplified while remaining completely general.
It is worthwhile to describe some of the fundamental differences between the characteristics of fiber-optic links, and fiber-optic components (excluding fiber). Fiber-optic links are generally very long (>50 km), and have very broad spectral features that vary on the scale of tens of nanometers. Because of the great lengths involved and high launch powers used to over come the loss, nonlinear interactions of light in the fiber are of significant interests. These nonlinear properties are not easily reduced to a simple transfer function matrix. In very long fiber-optic links such a submarine links, optical amplifiers are used to regenerate the signal and overcome loss. These amplifiers often operate in saturation which is another non-linear effect not captured in a simple transfer function matrix. Generally, fiber-optic links are not well characterized by simple transfer functions.
The relatively short path lengths (<1 m) within fiber optic components prevents any significant non-linear behavior. Because many passive fiber-optic components are filters used to separate channels closely spaced in wavelength, they are designed to have rapid variations in their properties, as a function of wavelength. Optical filters often display variations on the scale of tens of picometers instead of tens of nanometers as seen in fiber optic links. Not only does the transmission amplitude of these functions vary rapidly, but the phase response does as well, and so, in high bit-rate applications, the phase response must be accurately measured as well. Also, due to the fabrication techniques, many filters display some variation as a function of polarization. Again, in high bit-rate systems, this variation must be accurately characterized. It is in these last two measurements, optical phase, and polarization dependence, that the inventions presented here excel.
The underlying technology used was initially developed by Glombitza and Brinkmeyer, and published in 1993 (“Coherent Frequency-Domain Reflectometry for Characterization of Single-Mode Integrated-Optical Waveguides,” Journal of Lightwave Technology, Vol. 11, No. 8, August 1993). No patents were filed by either author with regard to this technology. Patents of a similar nature are U.S. Pat. No. 5,082,368 “Heterodyne optical time domain reflectometer,” which employed an acousto-optic modulator, and U.S. Pat. No. 4,674,872 “Coherent reflectometer,” which employed an optical phase shifter, and has expired due to nonpayment of maintenance fees. A co-pending U.S. patent application Ser. No. 09/606,120 filed Jun. 16, 2000 by Froggatt and Erdogan for an invention entitled “Single Laser Sweep Full S-Parameter Characterization of Fiber Bragg Gratings,” herein incorporated by reference, expands upon the published and unpatented work by Glombitza and Brinkmeyer to describe how the measurement of the phase and amplitude of the time domain response of a system can be used to determine the frequency domain (spectral) response of the system as well via an inverse Fourier transform. U.S. Pat. No. 5,896,193 for “Apparatus for testing an optical component” also describes an interferometer much like that described by Glombitza and Brinkmeyer although the patent filing on Feb. 14, 1997 post dates the Glombitza and Brinkmeyer article publication date of August 1993 by over three years.
The Jones Matrix Representation
An optical fiber component supporting two polarization modes can be fully described using a wavelength dependent Jones matrix. This matrix describes the transfer of energy from the device input to the device output, encompassing the full effects of polarization dependence. The Jones Matrix relates the input electric field to the output electric field by:
                              [                                                                      E                                      s                    ,                    out                                                                                                                        E                                      p                    ,                    out                                                                                ]                =                              [                                                            a                                                  b                                                                              c                                                  d                                                      ]                    ⁡                      [                                                                                E                                          s                      ,                      in                                                                                                                                        E                                          p                      ,                      in                                                                                            ]                                              Eqn        .                                  ⁢        1            where ES and EP are complex electric field amplitudes of the two orthogonal fields used to represent the total electric field. These orthogonal fields may be any orthogonal state and linear states oriented at 90 degrees are the most common. However, left hand and right hand circular would be an equally valid choice. No particular pair of orthogonal states is needed for the present invention.
This four element two-by-two matrix is completely general and includes all aspects of the device behavior. If the four complex numbers (a, b, c & d) can be measured as a function of frequency, then the device has been completely characterized. It is therefore an object of this invention to measure these four quantities, and from this basic physical measurement derive any and all of the desired parameters describing the device.